Process for producing an elongated superconductor with a bismuth phase having a high transition temperature and a superconductor produced according to this process

ABSTRACT

Process for producing an elongated superconductor with a bismuth phase having a high transition temperature and a superconductor produced according to this process. An elongated superconductor with at least one conductor core made of high-T c  Bi-containing superconductor material with the 2212 or 2223 phase is to be manufactured. For this purpose, the cross section of a structure made of Ag matrix material and a precursor of the superconductor material is reduced. Subsequently the raw conductor thus obtained is annealed in an oxygen-containing atmosphere. According to this invention, a temperature variation between a higher temperature (T1) and a lower temperature (T2) is provided for the annealing. The higher temperature (T1) is at most 7 K above the decomposition temperature (Tz), and the lower temperature (T2) is at most 9 K below the decomposition temperature (Tz).

DESCRIPTION

Process for producing an elongated superconductor with a bismuth phasehaving a high transition temperature and a superconductor producedaccording to this process.

BACKGROUND OF THE INVENTION

The invention concerns a process for producing an elongated single- ormulti-core superconductor having at least one conductor core embedded inan Ag matrix, which conductor core has a bismuth-containingsuperconductor material with a 2212 or 2223 type high-T_(c) phase. Theprocess comprises the following steps:

A structure is prepared from the matrix material and at least one core,made of the precursor of the superconductor material,

the structure is converted to a raw conductor using a specialcross-section-reducing treatment, and

the raw conductor is subjected to an annealing process with controlledmelting in an oxygen-containing atmosphere to form the high-T_(c) phase.

The invention also concerns a superconductor manufactured by thisprocess.

A similar process and a superconductor produced by this process areknown from "Supercond. Sci. Technol.," Vol. 5, 1992, pp. 591-598.

Known superconducting metal oxide compounds with high transitiontemperatures T_(c) of over 77 K, which are therefore also referred to ashigh-T_(c) superconductor materials (for short: HTSL materials), includein particular cuprate based on bismuth material Bi--Sr--Ca--Cu--O(BSCCO) or B(Pb)--Sr--Ca--Cu--O (B(P)SCCO) system. In this system, twosuperconducting phases appear, which differ by the number ofcopper-oxygen lattice planes (layers) of the crystal unit cell. Asuperconducting phase with the approximate composition Bi₂ Sr₂ CaCu₂O_(8+y), (referred to as a 2-layer/85-K or 2212-phase) has a transitiontemperature T_(c) of approximately 85 K, while the transitiontemperature of a superconducting phase with the approximate compositionBi₂ Sr₂ Ca₂ Cu₃ O_(10+x), (referred to as a 3-layer or 100-K or 2223phase) is approximately 110 K.

Attempts have been made to manufacture elongated superconductors in wireor tape form with these HTSL materials. A process considered suitablefor this purpose is referred to as "powder-in-tube" technology, known inprinciple from the manufacture of superconductors with the traditionalsuperconducting material Nb₃ Sn (see, for example, German Auslegeschrift1 257 436). According to this method, a powder made from a precursor ofthe HTSL material, which in general contains little or none of thedesired superconducting high-T_(c) phase, is filled into a tubularcarrier or into a matrix made of normally conducting material, inparticular of Ag or an Ag alloy. The structure thus obtained is thenbrought to its final dimensions by means of forming processes, which maybe interrupted by at least one heat treatment. Then the wire- ortape-shaped raw conductor is subjected to at least one annealingoperation, performed at least partially in an oxygen-containingatmosphere, e.g., air, to adjust or optimize its superconductingcharacteristics or to form the desired high-T_(c) phase (see, forexample, "Supercond. Sci. Technol.," Vol. 4, 1991, pp. 165-171).

If a plurality of such tape- or wire-shaped high-T_(c) superconductorsor their conductor precursors are bundled together, conductors with aplurality of superconducting cores, referred to as multi-core ormulti-filament conductors, can be obtained, which offer a series ofadvantages for technical applications (see the article by M. Wilhelm etal. entitled "Fabrication and Properties of MultifilamentaryBiPbSrCaCuO-2223 Tapes" of the "International Symposium onSuperconductivity" (ISS'93), Hiroshima, Japan, Oct. 26-29, 1993).

It is known from the aforementioned reference from "Supercond. Sci.Technol.," Vol. 5 that the textural properties of such reaction-annealedtape conductor cores can be improved, thereby increasing the criticalcurrents, and the dependence of the critical current densities on themagnetic field can be reduced by partial melting and subsequentcontrolled crystallization of the ceramic. The corresponding process isknown as the PFDR (phase formation-decomposition-recovery) process.According to this process, a nitrate mixture of suitable composition iscalcined at 830° C. and, after reaction annealing at 845° C. in air, ityields basically the 2-layer compound and alkaline earth cuprate. Withthis reaction product, a tape-shaped raw conductor is prepared with asilver shell according to the powder-in-tube process and annealed at835° to 838° C. to form the 3-layer phase. A one-time short melting ofthe conductor core at 860° C. between the reaction annealing and a firstsecondary annealing results in a higher core density and a finedistribution of 2-layer residues and intergranular minority phases in a3-layer matrix. Thus "pinning centers" are obtained and increasedmagnetic field-independence of the superconductance at 77 K is ensured.Additional compacting and annealing at 838° C. is performed after thefirst secondary annealing at 838° C.

A similar process with a PFDR process is also disclosed in WO 93/22799.In this known process, the 2223 phase obtained in the conductorstructure is also briefly melted at approximately 860° C. andpost-annealed at approximately 839° C. to thus obtain a small proportionof the 2212 phase dispersed in the 2223 phase, which serves as a matrix.

Superconductor manufacturing on an industrial scale according to thePFDR process presents problems, however, because the adjustment of theoptimum degree of partial melting is extremely critical. The propervolume of melted material is, however, decisive for obtaining aconductor with high core density and a high current capacity. For agiven composition of the core ceramic, the melted volume depends on thetemperature and the annealing time, but also on the conductordimensions. The greater the core volume, the lower the meltingtemperature must be. Low-viscosity melt, which is required for quick andhomogeneous material distribution and thus pore-and contaminant-freegrowth, is produced in sufficient amounts only in a temperature rangewhere the superconducting phases in the B(P)SCCO superconductordecompose again. If a relatively high melting temperature (860° C.) isprovided, as in the case of the known PFDR process, there is the dangerthat the decomposition of the desired superconducting phase can nolonger be reversed by subsequent annealing at a relatively lowertemperature level.

SUMMARY OF THE INVENTION

The object of this invention is therefore to provide a process thatwould at least alleviate the aforementioned problems, in particularregarding the danger of decomposition of the superconducting phases.

This object is achieved according to this invention by providing atleast one annealing operation with a temperature that periodicallyfluctuates between a first, higher, temperature level and a second,lower, temperature level, with the first temperature level being at most7° C. over the decomposition temperature of the high-T_(c) phase to beformed and the second temperature level being at most 9° C. below saiddecomposition temperature. The decomposition temperature can bedetermined accurately by observing the first appearance of a Bi₂ Sr₂CuO_(x) -type (also known as single-layer) phase in the core material.

As in the case of the known PFDR process, the process of this inventionallows conductors with a relatively high core ceramic density to bemanufactured. In addition, it has been observed that the conductorsannealed with periodically fluctuating temperature, rather thanisothermally, have a relatively higher critical current density at thesame time as a high density. The desired superconducting Bi phase musttherefore be at least largely present in the final product. In addition,the overheating temperature in the process according to this inventionis advantageously lower and can be set accurately and reproducibly.

Advantageous embodiments of the process according to this invention aredescribed in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with thedrawings in which:

FIG. 1 is a schematic illustration of a cross section through a rawconductor to be annealed according to the present invention;

FIG. 2 is a graph which shows the temperature variation in an annealingprocess according to the present invention;

FIG. 3 is a graph which shows the temperature variation for a specificembodiment of the annealing process according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The process according to this invention for manufacturing an elongatedsuperconductor with bismuth (Bi) high-T_(c) superconductor material isbased on the powder-in-tube process, which is known per se. Elongatedcomposite bodies, such as wires and tapes, containing HTSL materialbased on the Bi--Sr--Ca--Cu--O material system, can be produced with theprocess. The phase known as the 2223 phase should preferably be presentin the HTSL material in a proportion higher than 50 wt. %. It ispossible to use only the five above-named elements of the materialsystem to produce such HTSL material. Since the material system selectedfor this invention only needs to constitute the basis of the HTSLmaterial, i.e., represents the basic type, the process according to thisinvention should also include the possible full or partial replacement,in a well-known manner, of some of the aforementioned elements withother elements from the same element group of the periodic system. Thus,for example, Bi can be partially replaced with Sb or by Pb, whichparticularly favors the formation of the 2223 phase; Ba may beconsidered, for example, to replace the alkaline earth metals Sr and Ca.Furthermore, Cu can be partially substituted by smaller amounts of othermetals, such as Fe, Co, Ni, or Al. In addition, it should be possible tointroduce additives, serving to improve the reaction mechanism, in thematerial system. Thus, for example, it is known that Ag or Ag₂ Oadditions actively participate in the reaction of formation of thedesired high-T_(c) phase, but are not incorporated in the crystalstructure of this phase. However, we shall consider an HTSL materialcontaining six components, Bi, Pb, Sr, Ca, Cu, and O, for theembodiment, with unavoidable impurities of the individual componentsnecessarily included.

To manufacture an HTSL material precursor in a powder form, thewell-known initial materials will be used, which make the formation ofthe 110 K or 2223 phase possible. In order to ensure the properstoichiometry of this high-T_(c) phase, oxide and/or carbonate powdersof the individual components of the Bi--Pb--Sr--Ca--Cu--O materialsystem, for example, Bi₂ O₃, PbO, SrCO₃, CaO, and CuO powder, areprepared in a proportion of 1.8:0.4:2.0:(1.8 to 2.2):3.0:10.3 betweenthe individual components. Small variations in the composition of thepowder mixture in relation to the stoichiometric relationship in thesuperconducting high-T_(c) phase to be formed should be possible ingeneral, as long as an at least partial formation of the desired phaseis ensured. Said powder mixture is then calcined, in the well-knownmanner, in two stages, by annealing it for 3 to 4 hours at approximately800° C. and subsequently, for example, for 16 hours at approximately820° C. The material thus obtained is then ground, for example, in aplanetary ball mill. The product obtained is the "precursor" or"calcine" for the HTSL material and exhibits a plurality of completelydifferent compounds of the HTSL material, for example, alkaline earthpotassium plumbate (Ca,Sr)₂ PbO₄, alkaline earth cuprates of differentcompositions (Ca,Sr)_(x) Cu_(y) O_(x+y), CaO, CuO, as well as perovskite(Bi,Pb)₂ Sr₂ CuO_(x) (also known as 1-layer) and (Bi,Pb)₂ Sr₂ CaCu₂O_(y) (also known as 2-layer).

The HTSL material precursor thus produced is subsequently introduced ina tubular carrier made of a special matrix material and ispre-compacted. Ag and Ag alloys are advantageously used as matrixmaterials, since these materials allow oxygen transport throughdiffusion mechanisms, in particular at high temperatures.

The structure thus obtained, consisting of the tubular carrier and theprecursor material core contained in it, is subsequently subjected to aspecial forming operation to reduce the cross section and a specialannealing operation to form the desired 2223 phase. In general, asequence of a plurality of forming treatments are required to form thedesired end shape of the conductor and a plurality of annealingoperations are required to form the high-T_(c) phase of thesuperconducting material, at least one of which must be carried outaccording to this invention. At least one forming treatment, forexample, uniaxial pressing, can be performed between several annealingoperations to achieve in particular better texturization of thesuperconducting material.

For the at least one forming operation, all known procedures, such asfor example pressing, rolling, calendering, hammering, and drawing, canbe considered, which can also be used in combination. This treatment canbe performed both at room temperature and at higher temperatures. Thepressure exerted on the core made of the treated precursor material isadvantageously adjusted to between 3 and 20 kbar, preferably between 5and 10 kbar. A high-density conductor core is thus obtained in an Agmatrix. After the at least one forming operation, a raw conductor in theform of a composite is obtained with a configuration at least largelycorresponding to the desired end product, preferably in the form of atape.

With the above-described process steps involving calcination to obtain asuperconductor material precursor and forming of this precursor into acarrier tube made of the matrix material, not only single-coreconductors can be manufactured. Rather, these process steps can alsoserve as a basis for manufacturing multi-core, or multifilamentary,conductors using a bundling method that is well-known per se. Accordingto this method, a plurality of carrier tubes, each containing a core ofcalcined precursor material, can be bundled into a shell, in particular,a shell made of the matrix material. This structure is then formed atleast once, which results in a multicore raw conductor. Of course,pre-formed and/or possibly pre-annealed single-core raw conductors canalso be introduced in a suitable shell and then further processed into amulticore raw conductor using at least one forming operation.

One embodiment of such a multicore raw conductor obtained after asequence of pressing and rolling operations to be processed into atape-shaped superconductor according to the invention is illustrated inFIG. 1 of the drawing as a cross-section. The raw conductor is generallydenoted with 2. Its (for example 19) conductor cores 3i (with 1≦i≦19)made of the precursor material are embedded in a matrix 4 made of Ag.Raw conductor 2 has, for example, the following dimensions that aretypical for a tape:

Tape thickness D: 100 μm to 500 μm preferably 200 μm to 350 μm;

Tape width B: 2 mm to 6 mm preferably 3.5 mm to 4.5 mm;

Conductor core thickness d: 10 μm to 50 μm preferably 20 μm to 35 μm;

Conductor core width b: 30 μm to 200 μm preferably 150 μm to 180 μm.

The corresponding typical dimensions for a tape-shaped single-core rawconductor are:

50 μm≦D≦400 μm, preferably 100 μm≦D≦200 μm;

1.5 mm≦B≦5 mm, preferably 2 mm≦B≦3 mm;

10 μm≦d≦40 μm, preferably 20 μm≦d≦30 μm

500 μm≦b≦4.5 mm, preferably 1.5 mm≦b≦2.5 mm

A raw conductor, for example, with a structure as illustrated in FIG. 1in general does not yet exhibit the desired superconducting properties.This means that the desired superconducting phase, in particular of the2223 type, is not yet present in its at least one core with sufficientpurity and a texture allowing for a high critical current density.Therefore, the raw conductor is subjected to at least one specialannealing operation, with a periodically, oscillatina, non-isothermaltemperature variation, according to the invention. The periodicallyoscillating temperature variation occurs between two extreme values oftemperature in prescribed, regularly returning time intervals orperiods. In every period, a maximum value and a minimum value oftemperature are set one single time. Annealing is carried out in anoxygen-containing atmosphere, for example, in air or the like with thepartial pressure of oxygen between 1 and 200 mbar.

FIG. 2 schematically shows this temperature variation in a diagram,where the annealing temperature T (in 1° C. steps) is plotted againsttime t (in arbitrary units).

T1 is a first temperature at a higher temperature level,

T2 is a second temperature at a relatively lower temperature level,

ΔT (=T1-T2) is the temperature difference (oscillation amplitude)between the two annealing temperatures T1 and T2,

Δt is the duration of an oscillation period, consisting of the times ofthe two annealing periods at temperature T1 and temperature T2,

(dT/dt) is the average heating rate from temperature T2 to temperatureT1, and

(dT*/dt) is the average cooling rate from temperature T1 to temperatureT2.

Temperature T1 is higher than the decomposition temperature Tz of thehigh-T_(c) phase to be formed by so much that during the annealingperiods at the higher temperature T1 a low-viscosity melt is obtainedthat can penetrate to all points of the texture between the individualphases and ensures the particle transport to these points that isrequired for the reaction. Accordingly, the superconducting phases oftype 2212 and/or 2223 will melt or start to decompose at the highertemperature T1. In the following annealing period with the reducedtemperature T2 (below Tz), the superconducting phases can then recoverand grow in all directions, since they are wetted by the melt that isnow highly viscous and no particle transport is needed over greaterdistances. The temperature variation must be selected so that over timemore superconducting phase is produced during the periods with the lowerannealing temperature T2 than is lost during the periods with the higherannealing temperature T1. The dwell times at the different temperaturelevels can be different. In addition, it must be ensured that the firstannealing of the raw conductor starts with a period of lower annealingtemperature as a "reaction annealing," because when the raw conductor isfirst heated to a high temperature, excessive melting of the precursormaterial would hinder the formation of texturized, lead-containingBi-2212 crystals on the boundary surface with the Ag material of thematrix. These crystals are essential as seeds for the formation of aproper texture in the entire superconducting core. A 1-layer phase thatmay still be present quickly disappears during the first heating to ahigh temperature at the latest, namely at a temperature prior toreaching the melting point Tz for the first time.

The following non-isothermal temperature variation is provided for thereaction and recovery annealings according to the invention:

The first higher temperature T1 must be at most 7 K, in particular atmost 5 K, more advantageously at most 3 K above the decompositiontemperature Tz of the ceramic superconductor material in the matrix madeof Ag material. The decomposition temperature Tz is, by definition, thetemperature at which the formation of the Bi-2201 phase, not previouslyor no longer present can be detected, using X-ray diffractometry, in theceramic core after quick cooling of the conductor end product (silvertape conductor). Depending on the partial pressure of oxygen and theconductor geometry, Tz is between approximately 800° C. and 880° C. Thesecond, lower annealing temperature T2 must be at most 9 K, preferablyat most 7 K, more preferably at most 5 K below the decompositiontemperature Tz. In general, an average annealing temperatureTm=(T1)/2+(T2)/2 is selected between T2 and T1, which is 0.5 K to 3 Kbelow the decomposition temperature Tz of the ceramic superconductingmaterial.

The amplitude ΔT of the annealing temperature variation (oscillation)must be between 1 K and 10 K.

The duration of an oscillation period Δt must be between 2 minutes and200 minutes.

The temperature variation over time can advantageously be described by asinus function or a trapeze function with heating and cooling rates(dT/dt) and dT*/dt) between 10 K/min and 0.01 K/min in atemperature-time diagram (see FIG. 2).

In a diagram similar to that of FIG. 2, FIG. 3 shows a temperaturevariation according to the invention for a specific embodiment of asingle-core raw conductor, where the periodic temperature variationstarts at time t_(o) and temperature T2. A silver tube with 8 mm outerdiameter and 1 mm wall thickness is used for manufacturing the rawconductor filled with a calcine having the nominal composition Bi₁.84Pb₀.35 Sr₂.0 Ca₂.1 Cu₃.0 O_(10+x) and hammered to a diameter of 0.5 mm.After being inserted in a second silver tube with the same dimensions (8mm O.D., 1 mm wall thickness), the assembly is hammered to a diameter of1.5 mm, then drawn to a diameter of 1.3 mm, and finally rolled to a tapewith 0.1 mm thickness. The raw conductor is then subjected to tripleannealing in air in a tubular furnace, as illustrated in FIG. 3 andpressed uniaxially under a pressure of 10 kbar for compacting andtexturizing after each annealing operation. The following parameterswere used for the individual annealing operations:

    T1=841.4° C., T2=838.1° C., Tm=839.75° C.

    ΔT=3.3 K

    (dT/dt)=(dT*/dt)=0.3 K/min

    ΔT=22 min.

In this case the decomposition temperature was approximately 840° C. Thesuitably annealed end products had reproducibly critical currentdensities clearly higher than 4×10⁴ A/cm² at zero field and 77 K.

For the above-described embodiments, it was assumed that the elongated,in particular tape-shaped, superconductors were to be annealed accordingto the invention in a powder-in-tube process. The invention is, however,not limited to such a process. It is equally well suited formanufacturing superconductors with a Bi-2212 phase or Bi-2223 phasehaving Ag shells assembled subsequently around a superconductor materialprecursor. An example of such an assembled shell is given in GermanOffenlegungsschrift 43 08 681. According to that publication, theprecursor is to be applied in a paste form onto a tape-shaped Agcarrier. The surface portions of this precursor that remain free arethen sealed using, for example, a foil-type covering means prior toprocessing the structure thus obtained into a raw conductor, which is tobe subsequently annealed according to the invention.

The process according to the invention is also not limited to themanufacture of single- or multicore superconductors basically only withthe Bi-2223 phase. Superconductors having only mainly Bi-2212 phase or amixture of Bi-2212 and Bi-2223 phases can also be produced with thisprocess. The aforementioned definition of the decomposition temperatureTz applies to those cases as well.

What is claimed is:
 1. A process for producing an elongated single- or multicore superconductor with at least one conductor core embedded in a matrix consisting of Ag material that has a bismuth-containing superconductor material with a high-T_(c) phase of 2212 and/or 2223 type, comprising the steps of:preparing a structure of matrix material and at least one core made of a superconductor material precursor, converting said structure to a raw conductor, and subjecting the raw conductor to an annealing operation with controlled melting in an oxygen-containing atmosphere to form the high-T_(c) phase, wherein at least one annealing operation is provided with a oscillating temperature variation, said oscillating temperature variation being performed in a plurality of time periods, in each of said time periods the temperature varying between a first, higher temperature level (T1) and a second, lower temperature level (T2), with the first temperature level being at most 7 K above the decomposition temperature (Tz) of the high-T_(c) phase to be formed and the second temperature level (T2) being at most 9 K below the decomposition temperature (Tz), wherein a temperature difference (ΔT) of between 1 and 10 K between the higher and the lower temperatures T1 and T2 is provided.
 2. The process according to claim 1, wherein a higher temperature (T1) of at most 5 K above the decomposition temperature (Tz) of the high-T_(c) phase is provided.
 3. The process according to claim 1, wherein a lower temperature (T2) of at most 7 K below the decomposition temperature (Tz) of the high-T_(c) phase is provided.
 4. The process according to claim 1, wherein an oscillation period of between 2 minutes and 200 minutes is provided.
 5. The process according to claim 1, wherein an average heating rate (dT/dt) from the lower temperature T2 to the higher temperature T1 and an average cooling rate (dT*/dt) from the higher temperature (T1) to the lower temperature (T2) of between 0.01 K/min and 10 K/min are provided.
 6. The process according to claim 1, wherein the raw conductor is annealed to form the high-T_(c) phase at a partial pressure of oxygen of between 1 mbar and 200 mbar.
 7. The process according to claim 1, wherein to produce the precursor material, at least one of the metallic components of the aforementioned type of high-T_(c) phase is substituted with at least one other metallic component.
 8. The process according to claim 1, wherein the structure is produced from the matrix material and the at least one core made of the precursor material by the powder-in-tube process.
 9. The process according to claim 1, wherein a plurality of annealing operations are provided, between which the conductor is subjected to a uniaxial pressing treatment.
 10. The process according to claim 1, wherein said step of converting further comprises using a cross-section-reducing forming operation.
 11. The process according to claim 2, wherein a higher temperature (T1) of at most 3 K above the decomposition temperature (Tz) of the high-T_(c) phase is provided.
 12. The process according to claim 3, wherein a lower temperature (T2) of at most 5 K below the decomposition temperature (Tz) of the high-T_(c) phase is provided.
 13. The process according to claim 7, wherein the Bi component is partially substituted with Pb and/or a stoichiometry that is different from the basic composition of the high-T_(c) phase is provided, as long as at least a partial formation of the high-T_(c) phase is ensured. 